Perhaps it need not be said that one vagina is not the same as the next, but medically speaking, doctors have long thought that all “healthy” vaginas had certain things in common — namely levels of some good bacteria.

But a new study led by Jacques Ravel at the University of Maryland School of Medicine reports that in fact not all women are created equal. The vaginal microbiome — the community of bacteria living in the vagina — varies considerably between women, the study found, and even within the same woman at different times.

The results, published this week in Science Translational Medicine, suggest that there may not be a single standard for a “normal” or “healthy” vaginal environment. Levels of bacteria that may signify bacterial infection in one woman may be healthy in another. The study involved 32 women who submitted vaginal bacterial swabs taken twice a week for 16 weeks.

“We were surprised at the extent of differences we saw between women, and the uniqueness of patterns observed with various individuals,” says Larry Forney, director of the Institute for Bioinformatics and Evolutionary Studies at the University of Idaho and one of the co-authors of the paper. “When you look at the 32 women, it’s hard to find two who are the same when it comes to their vaginal microbiota.”

It’s the first study to document such dynamic differences, and could change the way doctors currently diagnose and treat vaginal infections. Yeast and bacterial infections affect about 25% to 30% of women in the U.S. on any given day. Rather than using a one-size-fits-all approach to treatment, each woman may require more tailored treatment.

Previous studies of the bacterial communities found in the vagina have relied only on samples taken at one point in time. Ravel and his colleagues took a series of samples because they wanted to learn more about changes to the bacterial communities over time: Do they fluctuate consistently? Do they differ between women? What external factors, like sexual intercourse or exposure to compounds, affect the makeup of the bacterial communities? Are certain microbial recipes better at warding off infections?

Scientists have known that in general that certain bacteria living in the vagina — Lactobacillus bacteria —can help combat yeast infections by releasing lactic acid and other acidic compounds that kill yeast and other bugs. But no studies had looked at the vaginal communities over time, and none had applied the latest genomic technology to DNA fingerprint the types of bacteria living in the organ.

Ravel and colleagues’ genomic analysis confirmed the existence of five main groups of bacterial communities in the vagina, but revealed for the first time that not all women harbor the same breakdown of these populations. In fact, even among the small number of women in the study, the researchers found great variation by race: Hispanic and African-American women tended to have microbiomes that are not thought to protect against infection as effectively as the bacteria that were more commonly found in white and Asian women.

But a bacterial makeup that may put one woman at risk of infection may just be a normal state for another. Similarly, even within the same woman, changes in the microbiome over time may be normal.

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These results may help doctors individualize the diagnosis and treatment of vaginal infections. Currently, doctors treat all infections with the same antibiotics, as if they were caused by the same bacterial problems. The results aren’t ideal. On average, about 70% of women who are treated for bacterial vaginal infections will experience a recurrence, says study co-author Rebecca Brotman, assistant professor of epidemiology and public health at the Institute for Genome Sciences at the University of Maryland.

“With the microbiome of the vaginal communities, we can start thinking about personalized medicine for women,” says Ravel.

Alice Park is a writer at TIME. Find her on Twitter at @aliceparkny. You can also continue the discussion on TIME’s Facebook page and on Twitter at @TIME.